1. Field of the Invention
The present invention relates to a projection exposure apparatus, and more particularly to an alignment mechanism used in the projection exposure apparatus and an optical system used in the alignment mechanism.
2. Related Background Art
Recently, reduction projection exposure apparatus of the step-and-repeat method (steppers) are frequently used as apparatus for transferring fine patterns onto a photosensitive substrate (a semiconductor wafer coated with a photoresist layer) with high resolution in the photolithography process. With an increase in degree of integration of semiconductor elements, it becomes necessary that the steppers of this type use shorter-wavelength exposure light and that a projection lens with higher numerical aperture (N.A.) be developed. The resolving line width has reached the submicron level (about 0.5 .mu.m) on wafer. In order to transfer such high-resolution patterns, it is necessary to achieve positioning between a pattern on a mask or reticle (hereinafter referred to as reticle) and a shot area on a wafer with alignment accuracy matching with the resolution (normally about one fifth of the resolving line width). As an alignment method for such steppers, for example, the TTR (Through The Reticle) method is conventionally well known, which is arranged to simultaneously detect an alignment mark formed around the circuit pattern of reticle and an alignment mark formed around the shot area on wafer.
An alignment mechanism of the TTR method is so arranged that an alignment mark on reticle (hereinafter referred to as a reticle mark) and an alignment mark on wafer (hereinafter referred to as a wafer mark) both are optically detected with high accuracy to obtain a relative positional deviation amount between them and that the reticle or the wafer is finely moved to correct the deviation amount.